Electronic system for generating and controlling light effects on projectors

ABSTRACT

Electronic system for generating and controlling light effects on projectors utilizing arc lamps, composed of an electronic power unit provided with a transformer fed via transistors, the secondary circuit of the transformer being connected to the feed terminals of the lamp, within the secondary circuit of the transformer there being connected a component for withdrawing a proportion of the instantaneous current circulating through the secondary circuit to produce an electrical signal which is made available to a control block which also receives an electrical reference signal generated by a pulse generator, the control block being arranged to compare the two electrical signals and to set the transistors to a conducting or inhibiting state depending on the result of the comparison, the amplitude and frequency of the electrical reference signal being adjustable by the operator.

REFERENCE TO RELATED APPLICATIONS

The present application is the national stage under 35 U.S.C. 371 of international application PCT/EP99/07779, filed Oct. 08, 1999 which designated the United States, and which international application was published under PCT Article 21 (2) in the English language.

TECHNICAL FIELD

This invention generally concerns an electronic system for controlling the power fed to light beam projectors, particularly of medium and high power, which use short arc lamps such as xenon and metal halide lamps. More specifically, the invention relates to an electronic control system which enables both a stroboscopic effect and a dimmer effect to be obtained with said type of lamp.

BACKGROUND ART

The known art teaches that with this type of lamp a stroboscopic effect, ie, a rapid variation in light intensity, and a dimmer effect, ie, an attenuation in light intensity, can only be obtained by using mechanical means.

To achieve a stroboscopic effect, devices are known comprising vanes which are made to oscillate in a plane perpendicular to the light beam so as to obscure the beam. The maximum vane oscillation frequency which can be achieved-with devices of this type is of the order of 9 Hz for small and medium dimension projectors. As the projector dimensions increase the maximum vane oscillation frequency decreases. The ideal frequency at which the human eye perceives a stroboscopic effect lies between 7 and 10 Hz, which values can hence be approached only by small dimension projectors. In contrast, to achieve a dimmer effect, movable mechanical shutters are used located in front of the light beam in such a manner as to be able to partially or completely intercept their light flow.

A drawback of such shutters is that they are unable to completely intercept the light beam for a lengthy period. This is due to the considerable heat which develops on the shutter surface, so deforming it and making the shutter unusable.

An object of the invention is to overcome said drawbacks within the framework of a rational and economical solution.

DISCLOSURE OF THE INVENTION

The invention attains this and further objects by providing an electronic system for controlling the power fed to short arc lamps able to generate and control light effects such as a dimmer effect and a stroboscopic effect.

The electronic control system of the invention is able to control the instantaneous power transmitted to the lamp and to vary it with such a frequency as to achieve a stroboscopic effect at the ideal frequency perceivable by the human eye, or to achieve a dimmer effect.

More specifically, said system comprises an electronic power unit of switching type controlled by a feedback system which causes it to deliver power on the basis of a reference value which can oscillate between zero and the maximum allowable lamp power. Said reference value can be set by the operator or be generated by a suitable pulse generator.

To better clarify the constructional and operational characteristics of the invention a preferred embodiment thereof is described hereinafter by way of non-limiting example and illustrated on the accompanying drawings.

FIGS. 1A, 1B and 1C illustrate the electronic power and control circuit of the invention.

The figures show the lamp 1 (with relative firing device), which is fed by a power block 2. Said power block 2 is connected to the electrical mains supply by way of a usual rectifier 3.

Specifically, the power block 2 comprises a transformer TR1 the primary winding of which is pulse-fed by four identical MOSFET transistors TM1, TM2, TM3, TM4. The conduction or inhibition of the four transistors TM1, . . . , TM4 is controlled by the control block 4.

A normal measurement transformer TR2 is connected into the secondary circuit of the transformer TR1 to withdraw a small part of the instantaneous current circulating through the secondary winding of the transformer TR1 in order to obtain a voltage value therefrom and feed it to the control block 4. The control block 4 is therefore connected to the power block 2 in such a manner as to provide first order feedback follower control. The control block 4 comprises a PWM (pulse width modulator) electronic card, which in the illustrated example is of type L4990 manufactured by Thomson, its purpose being to compare the voltage value corresponding to the instantaneous current fed to the lamp 1 with a reference value generated by the pulse generator block 5. In detail, the generator block 5 is formed from an astable-connected timer LM555 generating voltage pulses at a frequency of between 3 and 19 Hz, which the user can vary by adjusting the trimmer PT2. The user can also vary the amplitude of the voltage generated by the timer LM555, which is done by adjusting the trimmer PT1. Between the output of the timer LM555 and the PWM card L4990 there is connected a signal adapter station 50, comprising two identical operational amplifiers LM358 connected as in FIG. 1C.

The PWM card L4990 compares the value of the instantaneous voltage signal withdrawn by the transformer TR2 with the voltage generated by the timer LM555. If the voltage generated by the timer LM555 exceeds the instantaneous voltage withdrawn by the transformer TR2 the PWM card L4990 increases the conduction time of the MOSFETS. If the opposite is the case, ie the instantaneous voltage fed to the lamp exceeds the reference voltage generated by the timer LM555, the PWM card L4990 decreases the conduction time of the transistors TM1, . . . , TM4.

The transistors TM1, . . . , TM4 are not controlled directly by the card L4990 but via an integrated circuit or driver IR2110. To achieve the stroboscopic effect, the operator adjusts the trimmer PT2 to set the pulse frequency generated by the timer LM555 to a value of between 8 and 12 Hz.

In contrast, to achieve the dimmer effect the operator closes the switch SW to short-circuit the resistor R22 in order to obtain a constant reference signal, and then adjusts the trimmer PT1 to decrease the amplitude of said reference signal.

The ensuing table summarizes all the technical characteristics of the constituent components of the invention shown in FIGS. 1A, 1B, 1C, and the relative connections.

Component Type Value Make RD Rectifier R1, R3, R5, R7, Resistor 22 KΩ R25, R26, R27 R2, R4, R6, R8 Resistor 15 KΩ R9 Resistor 18 R10, R20, R22 Resistor 4K7 R11, R15 Resistor 100 KΩ R12 Resistor 2R2 R13 Resistor 5R6 R14 Resistor 70 KΩ R16 Resistor 5K6 R17, R28 Resistor 47 KΩ R18, R24 Resistor 33 KΩ R19 Resistor 100 Ω R21, R23 Resistor 10 KΩ RV1 Variable Res. 10 KΩ RV2 Variable Res. 5K6 PT1 Trimmer 1 KΩ PT2 Trimmer 470 KΩ TM1, TM2, TM3, MOSFET Transistor STW 2ONB50 Sgs-Thomson TM4 T1 Transistor BC237B DZ1, DZ2, DZ3, Zener Diode 18 V DZ4 DZ5 Zener Diode 10 V 1 W D1, D2 Diode MUR 860 D3, D4 Diode RURG 8060 D5 Diode BAT 49 D6 Diode MUR 160 S Discharger V130 LA10 TR1 Transformer TR2 Transformer L Inductance C1 Capacitor 470 nF C2 Capacitor 100 pF C3, C6, C7 Capacitor 100 nF, 50 V C4, C5, C10 Capacitor 1 μF, 50 V C8 Capacitor 3n3 C9 Capacitor 10 nF CP1 Capacitor 470 μF, 200 V CP2 Capacitor 100 μF, 25 V IR2110 Driver International Rectifier L4990A PWM Electr. Card Sgs-Thomson LM555 Timer National LM358 Operational Amplif. SW Switch

It should be noted that the control system of the invention can also be used on readily available known power units. The invention can further be applied to projectors using alternating current arc lamps of metal halide type or the like. For this purpose it is sufficient to connect a DC/AC converter between the power unit output and the lamp to convert the current from direct to alternating. 

What is claimed is:
 1. An electronic system for generating and controlling light effects on a projector utilizing at least one arc lamp, comprising an electronic power unit provided with a transformer fed via transistors, a secondary circuit of said transformer being connected to feed terminals of said lamp, wherein within the secondary circuit of said transformer there is connected a means for withdrawing a proportion of the instantaneous current circulating through said secondary circuit to produce an electrical signal which is made available to a control block which also receives an electrical reference signal generated by a generator block, said control block being arranged to compare said two electrical signals and to set said transistors to a conducting or inhibiting state depending on the result of said comparison, in which the amplitude and frequency of said electrical reference signal are adjustable by trimmer means controlled by an operator to obtain light effects perceptible by human eyes.
 2. An electronic system as claimed in claim 1 characterized in that said control block comprises a PWM electronic card arranged to control the transistors by means of a driver circuit.
 3. An electronic system as claimed in claim 1, characterized in that said generator block comprises a pulse generator.
 4. An electronic system as claimed in claim 3, characterized in that said pulse generator is an oscillator.
 5. An electronic system as claimed in claim 3, characterized in that said pulse generator is a timer.
 6. An electronic system as claimed in claim 1, characterized in that the frequency of said electrical reference signal can be adjusted between 7 and 10 Hz to obtain a stroboscopic effect.
 7. An electronic system as claimed in claim 1, characterized in that the amplitude of said voltage reference signal can be adjusted to obtain a dimmer effect after rendering constant the generator block output signal.
 8. An electronic system as claimed in claim 1, characterized in that a DC/AC converter is connected between the output terminals of said electronic power unit and the feed terminals of the lamp to convert the direct current at the power unit output to alternating current. 